This invention relates to speed control of polyphase induction motors, and in particular, to an improved static control circuit for changing the effective rotor winding resistance of wound rotor motors.
Induction motors of the wound rotor type are commonly controlled by adjustment of the resistance of the secondary, i.e. rotor winding circuit. Such motors typically have a threephase rotor winding. One end of each phase winding is brought out to slip rings on the motor shaft. Desirable values of resistance are added in circuit with the rotor winding through stationary brushes in contact with the slip rings.
Speed control of a wound rotor motor is accomplished by adjusting this external rotor resistance to provide the torque necessary to maintain the desired speed for a given load. The maximum motor torque is independent of the resistance in circuit with the secondary winding. However, the slip at which the maximum torque occurs is proportional to secondary resistance. Thus, with a relatively high secondary resistance, maximum torque occurs at a high slip, i.e. at a relatively low rotor speed, so as to provide high starting torque efficiency. A reduction of secondary resistance results in a reduction of slip, and thus, an increase in full load speed. This type of control is referred to as "rotor current" or "secondary resistance control."
Prior art rotor current controls commonly utilized combinations of resistors which were selectively connected in circuit by electromechanical contactors to provide the desired external resistance magnitude. However, particularly when the motor must operate at a continuously variable speed, contactor tip wear and mechanical survival of the contactors are a problem. Additionally, excessive contactors are needed to obtain smoothly varying values of resistors over a substantial resistance range.
Static control devices, i.e. thyristors, such as silicon controlled rectifiers and triacs have been utilized in rotor current control circuits to overcome the above-recited problems associated with mechanical contactors.
A variable resistance effect may be generated merely by repetitively switching a thyristor in circuit with the rotor phase windings. However, this results in a current wave form varying substantially between zero and a very high value limited only by the effective motor resistance, and is likely to result in excessive torque pulses and rotor heating.
Accordingly, it is desirable to utilize thyristors in series circuit with load impedances, e.g. resistors. However, static control circuits, including static rotor current control circuits have unique design considerations, including as to gating and commutating of the solid state devices.
U.S. Pat. No. 3,529,224 -- Bedford, assigned to the assignee of this application, disclosed a number of static rotor control circuits wherein serially connected resistors and thyristors are arranged in delta and star configurations with the phase windings of a wound rotor motor. The anode-cathode circuits of the thyristors are in circuit with the phase windings. The thyristors are thus periodically cut off as the rotor winding phase current through the solid state device is cyclically reduced below a minimum holding current. The thyristors are thus line commutated, and no additional commutation circuits are required. The above-described circuits utilize one thyristor device per phase leg and employ phase control in which the thyristor is rendered conductive by the application of a gating signal at a selected phase angle of the alternating current signal induced in the rotor winding. Thus, during each time period during which the thyristor conducts, the thyristor is gated on at a desired phase angle intermediate the conduction cycle and conduction continues until the end of the conduction cycle when the current through the thyristor drops below the holding current. Gating signals for the thyristors in the three phase winding circuits are applied to the thyristors in sequence but at the same phase retard angle. Such phase control circuits may be quite complex, particularly because of the wide variations in rotor frequency occurring with changes in rotor speed. Modification of the phase retard angle from fully advanced firing results in increased ripple, i.e. cyclically occurring harmonics. Such harmonics are likely to cause undesirable motor heating and torque pulsations.
In an alternative static rotor current control system, also disclosed in the referenced Bedfore patent, the rotor winding voltages are rectified, such as by a diode bridge rectifier. The rectifier output is connected to a series circuit comprising a thyristor and a resistor, such that the thyristor controls current flow through the resistor. The thyristor is commutated by an external commutation and gating circuit so as to operate in a chopping or inverter mode. Typically, the chopping frequency is substantially greater than the rotor winding frequency and the pulse duty cycle is varied by time ratio control to adjust rotor current. While alleviating the above-described problems associated with phase control, such a system requires a forced commutation circuit, line voltage filtering, filtering of the chopping frequency by an inductor in the resistor-thyristor circuit, and a free-wheeling, or coasting, diode to discharge the filter inductor during intervals when the thyristor is cut off.
Accordingly, an object of the invention is to provide new and improved static speed control circuits for polyphase induction motors.
Another object is an improved static rotor current circuit for wound rotor motors of simple and economic construction.
A further object is an improved static secondary resistance control circuit for wound rotor motors designed to overcome the above-recited deficiencies of the prior art.